A blowing head for producing plastic film generally has an annular orifice from which a tube of thermoplastified synthetic resin is extruded so that, at a location downstream from the orifice this tube can be expanded or blown into a bubble or balloon, thereby reducing the wall thickness and forming a tube of the thermoplastic film.
It is known to provide a tubular device which can be affixed to the blowing head and which withdraws the blowing and cooling air through the inner tube while the film-blowing air is fed to the clearance between the device and the thermoplastic tube through a jacket or outer tubular member, being supplied to the latter from a source of the film-blowing air.
In the course of film blowing (see, for example German Patent Document DE-PD 38 20 530), a neck is formed between the film balloon and the plastic tube which is comparatively long and whose stability depends upon rheological and aerodynamic conditions.
In practice, it has been found that the neck region and tube tend toward instability when, for example, the device as described in the above-mentioned patent document has outlets for the film-blowing air only at the foot side of the device proximal to its connection to the blowing head and at its head side, i.e. remote from the blowing head.
The stabilization is here effected by balancing the amount of blowing air supplied with that which is withdrawn through the inner tube but in many cases, especially where the neck is comparatively long, instabilities still arise and improvement is required.